Prof. John Blackwell, Coffea Arabica Machina 101: PID, Part 1

This was dug up from the classic archives of Prof. John Blackwell, coffea arabica machina extraordinaire. An interesting read for those not familiar with or can use a refresher on the messy mechanics of how a PID controller works.



by John Blackwell

In an espresso machine we have an electric heating element in water and we are trying to control a given setpoint (maximum achieved temperature).

The THERMOSTAT is an electromechanical switch trying to control the on/off cycle of the heating element in water. The thermostat can turn off the heating element at the exact setpoint, but the heating element is still hot (it can not cool instantly) and continues to heat the water causing a temperature raise above the ideal setpoint (the problem). So what to do? Position the setpoint lower so it turns off sooner then rises to the desired maximum. Oh great now, at least the water did not get too hot but it has to cool down to the lower setpoint before it will turn on. The best we can do now is to have a thermostat with a very narrow bandwidth (the difference between on and off setpoints) and hope for the best in our cycle of heating and cooling.

PID can be described as a set of rules with which precisely regulates a closed loop control system. In our heating element in water example, the PID predicts when to control the on/off setpoint, making corrections so the heating element does not under or over shoot the desired temperature.

How PID works without the math? Closed loop control system means a method in which a real time measurement of the process being controlled is constantly fed back to the controlling device to ensure that the value which is desire is, in fact, being realized. The mission of the controlling device is to make the measured value, known as the process variable, equal to the desired value, usually known as the setpoint. The best way to accomplish this task is to use the control algorithm known as PID.

In its basic form, PID involves three mathematical control functions working together. The most important of these, Proportional control (the P), determines the magnitude of the difference between the setpoint and the process variable (known as the error), and then applies appropriate proportional changes to the control variable to eliminate the error. Integral control (the I) examines the offset of the setpoint and the process variable over time and corrects it when necessary. Derivative control (the D) monitors the rate of change of the process variable and makes changes to the output variable to accommodate unusual changes.

Each of the three control functions is governed by a user defined parameter. These parameters can be adjusted to optimize the precision of control. The process of determining the values of these parameters is know as PID Tuning, or BIG MATH!!

Remote Control Linea (from an iPhone)

For his home espresso machine, Mike Furlotti resurrected a Linea and modded it unlike any other mods I’ve never seen before. Mike purchased a non-functioning Linea from eBay, refurbed it, added PIDs to both boilers and preheated water inlets. Best of all, he embedded a web server running an MSP430 so he can control it from his iPhone. Why? So he can start the machine up and preheat it before he gets home. Monitoring and adjusting the temperature remotely is also built in.

GIZMODO Reports on La Marzocco (and Coffee)

Usually known for their reporting and humor on all things tech and gadgets, Gizmodo‘s Matt Buchanan wrote a comprehensive primer on coffee and basic brewing methods. Unlike most mainstream media, however, he actually sought knowledge and advice from those in the know – Ken Nye of Ninth Street Espresso and David Latourell of Intelligentsia. Matt also interviewed Jacob Ellul-Blake, La Marzocco s.r.l.‘s R&D, on what it takes for an espresso machine to brew a good espresso shot (other than a good barista, that is).

Also featured in the article is a picture of Ninth Street Espresso’s custom Linea Paddle.

Ninth Street Espresso’s Linea Paddle with individual coffee boilers, 3 PID controllers, preheated inlet water line, and barista lights.

Visions Espresso’s Coffee Enhancement Lounge

Our friends at Visions Espresso introduces their Coffee Enhancement Lounge (nice blurb here from Seattle Times), an extension of their services into training and consulting. Spearheaded by the always cheerful Sarah Dooley, every event should be a blast.

The Coffee Enhancement Lounge, or intentionally abbreviated as CEL so people like me won’t forget, features a La Marzocco Linea Manual Paddle, a standard Linea, and a GS/3. Soon it will also feature a Slayer, so baristas can have a smackdown between a Slayer and a Linea Manual Paddle.

La Marzocco espresso machines at the Coffee Equipment Lounge

When not training, Sarah and her accomplices at Visions enjoy water gun/balloon fights, pranks during coffee cupping, and anything else that doesn’t relate to work.

If you’ll be around Seattle on Saturday, August 29, check out CEL’s all day class/lecture on defect cupping, PID, SO espresso roasting, equipment mods, and more presented by many industry names you may know. The event will migrate over to a BGA sanctioned Barista Jam/Latte Art Smackdown at Victrola, where winning NW baristas will compete with LA baristas during Coffee Fest.